java String

1.indexOf()

    /**
     * Returns the index within this string of the first occurrence of the
     * specified substring.
     *
     * <p>The returned index is the smallest value <i>k</i> for which:
     * <blockquote><pre>
     * this.startsWith(str, <i>k</i>)
     * </pre></blockquote>
     * If no such value of <i>k</i> exists, then {@code -1} is returned.
     *
     * @param   str   the substring to search for.
     * @return  the index of the first occurrence of the specified substring,
     *          or {@code -1} if there is no such occurrence.
     */
    public int indexOf(String str) {
        return indexOf(str, 0);
    }

此函数源码的解释为：参数为要搜索的子字符串，返回值为指定子字符串第一次出现的字符串中的索引。如果不存在该字串则返回-1。